High vacuum pumps



United States Patent 3,2456% HIGH VACUUM PUMPS Alexander Rizo Rangabe, Stoneacre, Denmead, Portsmouth, England Filed Mar. 16, 1964, Ser. No. 351,922 Claims. (Cl. Mil-45) This invention relates to high vacuum pumps.

' In the well known Langmuir mercury vapour diffusion pump, a stream of vapour from mercury heated in a boiler is directed vertically upwards through a tube from which it is emitted against the interior of a conical shell. The conical shell forms, with the outlet of the tube, an annular jet for directing the vapour downwardly into the annular space between the tube and a surrounding vessel. The shell is spaced from this surrounding vessel to define an orifice forming an outlet from the space to be evacuated. Gas particles diifusing from this space through the orifice and into the downwardly directed vapour jet are entrained in the vapour. The surrounding vessel is cooled, for example by water, and the vapour condensing on the interior wall of this vessel is returned to the boiler which forms a reservoir for the mercury. The annular space between the tube and the surrounding vessel is connected to a backing vacuum, and the gas particles entrained by the stream of vapour are withdrawn into the pump creating this backing vacuum.

One disadvantage of the use of the Langmuir pump is that to achieve high vacua, cooled traps have to be used for condensing the mercury vapour. On the other hand, whilst it would be theoretically possible to use a very low vapour pressure pumping medium, such for example as liquid tin, extreme diificulties would be involved in maintaining the pumping mediumin its vapour phase at a sufi'iciently high pressure between the boiler and the vapour jet outlet since the entire apparatus would have to be raised to a temperature of the order of 1000" C.

It is an object of the invention to provide an improved high vacuum pump and in particular a pump which can use a very low vapour pressure pumping medium such as liquid tin.

The present invention consists in a high vacuum pump comprising a housing formed by a first part having an inlet from the space to be evacuated, and a second part having an outlet for connection to a backing vacuum, the two parts of the housing being interconnected through an orifice, a container within the housing for containing a liquid, a heater for boiling the liquid and a nozzle disposed adjacent the orifice and arranged to emit a jet of the vapour within said second part in a direction away from the orifice, the nozzle being arranged closely adjacent the surface of the liquid in the container and the heat emitted by said heater being localised in the liquid immediately adjacent the nozzle.

In order that the present invention can be fully understood two embodiments thereof will now be described, by way of example, with reference to the accompanying drawings, in which:

FIGURE 1 shows one form of apparatus according to this invention constituting a single-stage pump; and

FIGURE 2 shows another form of apparatus according to this invention constituting a two-stage pump.

Referring now to FIGURE 1, there is shown one form of apparatus according to the present invention comprising a conically shaped hood 1 located within an outer chamber 2, This hood is located with its free edge or base Patented Apr. 12, 1966 lowermost, and extends into an annular sump 3 in which a quantity of metal 4, to be vaporised, is retained and surrounds and seals the base of the hood. The sump drains into one limb of a U-shaped tube 5 whose other or second iimb is located coaxial with, and terminates adjacent the centre of curvature of, the sump. At the uppermost part of the hood 1, a tube 6 leads off to the backing vacuum, and a tube 7 adjacent the lower part of the hood communicates with a chamber (not shown) to be evacuated.

Conduction cooling fins 8 extend from the outer sides of the hood 1 to the inner walls of the chamber 2.

A heating coil 9 is disposed around the upper end of the second limb of the U-shaped tube and conductive leads 10 therefor are brought out through terminal seals 11. V

A port 1a communicates the inside of the hood 1 with the interior of the chamber 2 to reduce the pressure differences across the hood adjacent the sump.

The metal in the tube is maintained at boiling point by passing current through the heating coil, and by manometric effect, the level of the molten metal in the second limb is maintained just below the open upper end of this limb. This upper end thus forms an outlet nozzle 12 for the vaporised metal and may shaped in a convergent-divergent fashion, for example as a venturi, to accommodate a supersonic vapour stream.

The annular space between this nozzle 12 and the annular sump 3 defines an orifice through which gas particles can diffuse from the space within the chamber to be evacuated.

As the metal vapour given off from the boiling metal is discharged through the nozzle in the form of a jet, it is directed upwardly against the interior of the hood and entrains the gas particles diffusing through the orifice from the space below which consequently attains a high vacuum. The vapour which strikes the hood condenses on its inner surface and runs back into the annular sump 3 and thence back into the U-shaped tube 5. The crosssectional area of the upper part of the hood 1 and the mean free path of the metal vapour are such that the metal vapour is substantially wholly prevented from passing out along the tube 6.

The U-shaped tube may be made of a refractory material.

If the use of a heating coil is to be avoided, the U-shaped tube, or at least that part adjacent to and including the outlet nozzle, can be made of a conducting refractor material of a type which will not be attacked by the liquid metal or metal vapour, the material being heated by the passage of an electrical current through it. For example, the outlet may be formed by a double wall of which the inner wall is connected to the outer wall at the outlet lip of the tube, and the resistance heating can be concentrated at any particular position in the wall of the tube by suitably adjusting the resistance of the wall in that region.

Referring now to FIGURE 2 there is shown another form of apparatus according to the present invention, this apparatus being capable of attaining a higher vacuum than that attained by the apparatus shown in FIGURE 1.

This apparatus comprises two axially aligned conicaliy shaped hoods 1' and 1" located within an outer chamber 2'. The lower edge of the hood 1 depends into an annular sump 3' and the lower edge of the hood 1" depends into an annular sump 3". At the uppermost part of the hood 1' a tube 6' leads off to the backing vacuum and a tube 7' adjacent the lower part of the hood 1" communicates with a chamber to be evacuated.

Conduction cooling fins 8 extend from the outer sides of the hood 1, and conduction cooling fins 8 extend from the outer sides of the hood 1", to the inner wall of the chamber 2'.

Disposed coaxially within the annular sump 3' is an annular trough 13 made of a refractory material which communicates with the latter sump through a tube 14. Similarly, a refractory annular trough 15 is disposed c0- axially within the sump 3", this trough communicating with the sump 3 through a further tube 1.6. In addition, the sump 3' communicates With a sump 3" through a tube 17, for a purpose to be later described.

A quantity of metal 4' to be vaporised, is retained in the two sumps, each sump communicating with a separate one of the annular troughs. The metal in the annular trough 13 is maintained at boiling point by passing current through a heating coil 18 partially embracing this trough, conductive leads 19 for this heating coil being brought out through a wall 20 to the wall of the chamber 2' at terminal seals 21. Similarly, a heating coil 22 partially embraces the annular trough 15, conductive leads 23 connected to this coil being brought out through terminal seals 24. I

To facilitate the assembly and dis-assembly of the apparatus, the tube 1-4 is connected to a block 25 having an L-shaped bore communicating with the sump 3' and aligned with the tube 14 to provide a flow path for the metal. Similarly, the tube 16 is connected to a block 26 having an L-shaped bore communicating with sump 3" and aligned with the tube 16 to provide a flow path for the metal.

As before, by manometric etfect, the level of the molten metal in the annular troughs is maintained below the upper edges of these troughs so that the upper edges form outlet nozzles. The metal vapour given off from the boiling metal in the trough 15 isdischarged upwardly and cntrains gas particles diffusing from the chamber to be evacuated upwardly through the hood 1". In turn, the metal vapour given off from the boiling metal in the trough 13 entrains these gas particles diffusing through the upper end of the hood 1" and these particles are carried upwardly through the hood 1' to the tube 6'.

The vacuum created in the tube 7' is thus higher than that in the region of the trough 13 which in turn is higher than that in the tube 6.

This two-stage pump is therefore capable of creating a very high vacuum in the chamber to be evacuated and connected to the tube 7.

An additional advantage of the apparatus of this design over that shown in FIGURE 1 is that the two-stage pump avoids any obstruction in the path of the metallic vapour jets on which the vapour could be condensed and subsequently fall back into the high vacuum space.

Furthermore, the jets issuing from the annular troughs directly strike their associated hoods and thus more readily ensure the condensation of this vapour which consequently runs back into the sumps 3' and 3". However, since a small proportion of the metal vapour issuing from the trough 15 can diffuse into the upper hood 1' the tube 17 interconnecting the annular sumps 3 and 3" provides an overflow path for the metal in the sump 3' to the sump 3" to compensate for the loss of metal from the latter sump.

In order to ensure an adequate and highly localised heating of the annular troughs the heating coils are preferably initially wound fiat in a serpentine fashion and then wrapped around the base and sides of the troughs. Subsequently, they are secured to these troughs, and this may, for example, be etfected by initially cataphoretically coating the coil with alumina prior to the application to the trough and then further coating thev coil with alumina after the coil has been wrapped around these troughs.

The connecting leads 19, 23 to these coils may conveniently have a large-surface area plate-like member connected to them to act as a radiation flag to dissipate any heat conducted along these leads from the heated area.

The metal to be vaporised comprises a low melting point but high boiling point metal or alloy. Preferably, bismuth is employed, but alternatively tin, indium or lead, or their alloys, may be employed.

The annular troughs may conveniently be made from molybdenum or alumina.

In each of the above embodiments the heater coil may be made from tungsten and the hoods may be made from a material having a reasonably high thermal conductivity e.g. mild steel. 7

It is to be understood that the above embodiments may be modified without departing from the scope of the invention as defined by the appended claims. For example, the heat developed in the hoods may be suffi ciently low to obviate the need for the cooling tins 8, 8', a", and in order to still further localise the heat developed by the heating coils annular radiation shields may be provided, e.g. 37 (FIGURE 2). Furthermore, it is not necessary to employ heating coils to boil the metal, and alternatively this could be efiected by induction heating, electron bombardment or the passage of current directly through the troughor the metal itself.

By limiting the areas of the apparatus which are heated to a high temperature the apparatus according to this invention does not require an external liquid cooling sys tem as is commonly employed. Accordingly, the zip paratus is less bulky and the running costs are generally cheaper than previous pumps of comparable capacity.

Although, the invention has been described in connection with the vaporisation of a metal, it is alternatively possible to employ organic liquids which have a sufiiciently low vapour pressure.

I claim:

1. A high vacuum pump comprising a housing having a first part and a second part,

means in the first housing part defining an inlet from the space to be evacuated,

means in the second housing part defining an outlet for connection to a backing vacuum,

means defining an orifice between said first and second housing parts,

a container for containing a low vapour pressure liquid,

a heater for locally heating and vaporising the liquid in said container, and

a vapour-emitting nozzle disposed immediately adjacent the heater and said orifice for emitting a vapour jet Within said second housing part, the vapour jet having an upward component of motion.

2. A high vacuum pump comprising a housing,

means defining a first opening in said housing for connection to a gas-containing chamber to be evacuated,

means defining a second opening in said housing for connection to a backing-vacuum pump,

a container located between said first and second openings in said housing for containing a low vapour pressure liquid, said container comprising,

a nozzle positioned closely adjacent the surface of said liquid and directed towards the second opening; and

means for locally heating and boiling the liquid immediately adjacent the nozzle to direct a jet of vapour having an upward component of motion through said nozzle for entraining gas particles from said first opening, said vapour being subsequently condensed and flowing as a liquid downwardly under gravity to the container.

3. Ahigh vacuum pump comprising a housing,

a vessel mounted in said housing for retaining a liquid metal and enclosing a first passage,

a hood disposed over said vessel and having an upper part defining a restricted second passage and a lower part extending into the liquid metal in the vessel,

a nozzle located closely adjacent the surface of said liquid metal and directed into said hood,

a heater for locally heating and boiling the liquid metal immediately adjacent the nozzle and causing a jet of metal vapour to be emitted through said nozzle and into the interior of said hood,

means connecting said first passage to a chamber to be evacuated and means connecting said second passage to a backing vacuum pump.

4. A high vacuum pump according to claim 3, wherein said vessel comprises a first portion defining a reservoir for metal vapour condensing on said hood,

a second portion coaxial with and spaced inwardly from said first portion, said nozzle forming part of said second portion and a fluid flow duct interconnecting said first and second portions.

5. A high vacuum pump according to claim 4, wherein said first portion is annular and said hood comprises a conical shell.

6. A high vacuum pump comprising a housing,

means defining a first opening in said housing for connection to a gas-filled chamber to be evacuated,

means defining a second opening in said housing for connection to a backing vacuum pump and first and second cooperating pump units located between said openings for entraining gas particles from said chamber to the backing vacuum source, each said first and second pump unit comprising,

means defining an inlet orifice and means defining an outlet orifice,

a container for containing a low vapour pressure liquid disposed adjacent said inlet orifice a heater for locally heating and vaporising the liquid in said container, and

a vapour-emitting nozzle disposed immediately adjacent the heater for emitting "a jet of the vaporised liquid, and

means communicating the outlet orifice of said first pump unit with the inlet orifice of said second pump unit, the first opening communicating with the inlet orifice of the first pump unit and the outlet orifice of the second pump unit communicating with the second opening whereby the gas particles entrained by the jet emitted in said first pump unit are subsequently entrained by the jet emitted in said second pump unit.

7. A high vacuum pump according to claim 6, wherein each said first and second pump unit comprises,

condenser means for condensing the vaporised liquid emitted from said nozzle in its associated pump unit.

8. A high vacuum pump according to claim 7, wherein said condenser means comprises a hood, said hood having an upper part defining said outlet orifice of its associated pump unit and a lower part depending into the liquid in said container, at a position remote from said nozzle, for guiding said condensed vapour into said container.

9. A high vacuum pump according to claim 8, wherein each said container comprises -a first annular trough into which said lower part of the hood depends a second annular trough coaxial with and spaced inwardly from said first trough, said nozzle forming part of said second trough, and

liquid fiow-path means connecting said first and second annular troughs.

10. A high vacuum pump according to claim 9, wherein said heater comprises an electrically energised heater winding partially embracing an underside of said second annular trough and in good heat-conducting relationship therewith.

1: 1. A high vacuum pump according to claim 9, comprising a liquid-overflow tube connecting together said first annular troughs of the first and second pump units for compensating said first pump unit for any loss of vapour from said first pump unit to said second pump unit.

12. A high vacuum pump according to claim 11, wherein said liquid comprises a low melting point-high boiling point metal selected from the group of metals which comprises bismuth, tin, indium, lead and alloys of said metals.

13. A high vacuum pump comprising an outer housing,

an inner housing mounted within said outer housing,

said inner housing having an inlet duct for connection to a chamber to be evacuated, and

an outlet duct for connection to a source of backing vacuum pressure; and

a container for a low vapour pressure liquid, said container comprising an annular trough mounted adjacent said inner housing,

a nozzle spaced inwardly from said annular trough,

a U-shaped tube interconnecting said trough and said nozzle for maintaining a predetermined level of said liquid adjacent the outlet of said nozzle :by manometric efiect, and

a heater for locally boiling said liquid adjacent said nozzle,

said nozzle serving for emitting a jet of vapour towards said outlet whereby gas particles within said inner housing are entrained by said jet and are driven from said inlet to said outlet.

14. A high vacuum pump according to claim 13, wherein said inner housing comprises a frusto-conical hood for condensing said metal vapour and defining a first opening adjacent said outlet, defining a second opening, larger in cross-section than said first opening, adjacent said annular trough and defining a third opening adjacent said outlet, said third opening communicating with a space enclosed between said inner and outer housings for reducing the pressure differential between said space and a space enclosed by said inner housing.

15. A high vacuum pump comprising an outer housing,

an inner housing mounted within said outer housing,

said inner housing comprising,

an inlet duct for connection to a chamber to be evacuated and an outlet duct for connection to a backing vacuum a frusto-conical shell having an upper part communicating with said outlet duct and a lower part communicating with said inlet duct,

a vessel for containing a liquid metal said container comprising an annular trough-shaped reservoir mounted adjacent said inlet duct and having said lower part of the hood dependent therein,

a nozzle coaxial with and spaced inwardly from said annular reservoir and a fluid fiow duct interconnecting said reservoir and said nozzle for maintaining a predetermined level of said liquid metal adjacent the outlet of said nozzle by a manometric effect,

an electrical heater coil partially surrounding, and in engagement with, said reservoir for boiling the metal adjacent said nozzle, said nozzle comprising 7 8 means for emitting a jet of the vaporisedmetal into "References Cited by the Examiner said shell and towards said'outlet for entraining gas UNITED STATES PATENTS particles within said inner'housing from said inlet 1,434,851 11/1922 Smock et a1 230 10;1 to said outlet, r 3,134,534- 5/1964- Jancke et al. 230-10'1 an annular heat radiation shield interposed between 3,171,,584 3/ 1965 Hayashi 23045 said annular reservoir and said nozzle for localizing FOREIGN PATENTS the heat radiatedby said heater coil, and 60,178 2/ 1926 Sweden.

heat conductor means connected between and in coni tact with said shell and said outer housing for 'dis- 10 MARK Prmmy Exa'nmer sipating heat from said shell, WARREN E. COLEMAN, Examiner. 

6. A HIGH VACUUM PUMP COMPRISING A HOUSING, MEANS DEFINING A FIRST OPENING IN SAID HOUSING FOR CONNECTION TO A GAS-FILLED CHAMBER TO BE EVACUATED, MEANS DEFINING A SECOND OPENING IN SAID HOUSING FOR CONNECTION TO A BACKING VACUUM PUMP AND FIRST AND SECOND COOPERAING PUMP UNITS LOCATED BETWEEN SAID OPENINGS FOR ENTRAINING GAS PARTICLES FROM SAID CHAMBER TO THE BACKING VACUUM SOURCE, EACH SAID FIRST AND SECOND PUMP UNIT COMPRISING, MEANS DEFINING AN INLET ORIFICE AND MEANS DEFINING AN OUTLET ORIFICE, A CONTAINER FOR CONTAINING A LOW VAPOUR PRESSURE LIQUID DISPOSED ADJACENT SAID INLET ORIFICE A HEATER FOR LOCALLY HEATING AND VAPORISING THE LIQUID IN SAID CONTAINER, AND A VOPOUR-EMITTING NOZZLE DISPOSED IMMEDIATELY ADJACENT THE HEATER FOR EMITTING A JET OF THE VAPORIZED LIQUID, AND MEANS COMMUNICATING THE OUTLET ORIFICE OF SAID FIRST PUMP UNIT WITH THE INLET ORIFICE OF SAID SECOND PUMP UNIT, THE FIRST OPENING COMMUNICATING WITH THE INLET ORIFICE OF THE FIRST PUMP UNIT AND THE OUTLET ORIFICE OF THE SECOND PUMP UNIT COMMUNICATING WITH THE SECOND OPENING WHEREBY THE GAS PARTICLES ENTRAINED BY THE JET EMITTED IN SAID FIRST PUMP UNIT ARE SUBSEQUENTLY ENTRAINED BY THE JET EMITTED IN SAID SECOND PUMP UNIT. 